.box {
    position: relative; 
    margin: 30px;
    width: 450px;
    display: inline-block;
}
.img {
     width: 450px;
     height: 450px;
     border: 1px solid #333;
     position: relative;
     background: url(./imgs/1.jpg) no-repeat;
    }
.img > .mask {
    width: 200px;
    height: 200px;
    background-color: yellow;
    opacity: .4;
    position: absolute;
    top: 0px;
    left: 0px;
    display: none;
}
ul {
    display: flex;
    list-style:none;
    margin-top:10px;
}
ul > li {
    width: 54px;
    height: 54px;
    border: 1px solid #333;
    margin-right: 10px;
    cursor: pointer;
}
ul > li.active {
    border-color: red;
}
.enlarge {
    width: 400px;
    height: 400px;
    position: absolute;
    top: 0;
    left: 101%;
    background: url(./imgs/1.big.jpg) no-repeat;
    background-size: 800px 800px;
    background-position: 0 0;
    display: none;
}